An exhaust system of a diesel engine that comprises a diesel particulate filter (DPF) is capable of physically trapping diesel particulate matter (DPM) in exhaust gas passing through the exhaust system from the engine. DPM includes soot or carbon, the soluble organic fraction (SOF), and ash (i.e. lube oil additives etc.). The trapping of those constituents by a DPF reduces the amount of DPM entering the atmosphere, preventing what is sometimes seen as black smoke billowing from a vehicle's exhaust pipe.
When a DPF is present in the exhaust system of a motor vehicle powered by a diesel engine, it is desirable to regenerate the DPF to remove trapped soot before the accumulation of soot begins to interfere with engine and vehicle performance. Regeneration can typically be performed however only when conditions are suitable for effectively burning the trapped soot without undesired side effects.
Conditions that are conducive to successfully regenerating a DPF may not be present in all geographic regions for one or more various reasons. In the absence of such conditions, use of a DPF may be restricted to warm climates and then only as a passive exhaust treatment device. Such limitations hinder a more widespread use that is obviously desirable in order to maximize the benefit of DPF technology in motor vehicles.
CDPF technology extends DPF technology by including a catalyst in association with a DPF. One known CDPF exhaust treatment system comprises an oxidation catalyst disposed upstream of the DPF. The oxidation catalyst oxidizes hydrocarbons (HC) to CO2 and H2O and converts NO to NO2. The NO2 oxidizes carbon trapped in the DPF. While O2 could be used to oxidize DPM, the high temperatures needed for oxidation make O2 more difficult for treating diesel engine exhaust without the aid of still another catalyst such as cerium-oxide (CeO2). The inclusion of a second catalyst separate from the DPF adds to the cost of the exhaust treatment system.
Another known CDPF exhaust treatment device, sometimes referred to as a Catalyzed Soot Filter (or CSF), comprises an additional CeO2 catalyst in the DPF, eliminating the need for an upstream oxidation catalyst. This can reduce the overall size of a DPF and avoid the greater pressure drops present in a two-substrate DPF like the first type described above. In both types of DPF, the oxidation catalyst oxidizes hydrocarbons (HC) and converts NO to NO2, with the NO2 then being used to oxidize the trapped carbon.
The rate at which trapped carbon is oxidized to CO2 is controlled not only by the concentration of NO2 or O2 but also by temperature. Specifically, there are three important temperature parameters for regeneration.
The first is the oxidation catalyst's “light off” temperature, below which catalyst activity is too low to oxidize HC. That temperature is typically around 180° C.-200° C.
The second controls the conversion of NO to NO2. This NO conversion temperature spans a range of temperatures having both a lower bound and an upper bound, which are defined as the minimum temperature and the maximum temperature at which 40% or greater NO conversion is achieved. The conversion temperature window defined by those two bounds extends from approximately 250° C. to approximately 450° C.
The third temperature parameter is related to the rate at which carbon is oxidized in the filter. Reference sources in relevant literature call that temperature the “Balance Point Temperature” (or BPT). It is the temperature at which the rate of oxidation of particulate, also sometimes referred to as the rate of DPF regeneration, is equal to the rate of accumulation of particulate. The BPT is one of the parameters that is especially important in determining the ability of a DPF to enable a diesel engine to meet expected tailpipe emissions laws and/or regulations.
Typically, a diesel engine runs relatively lean and relatively cool compared to a gasoline engine. That factor makes natural achievement of BPT problematic. Therefore, a manufacturer of a DPF for a diesel engine should strive for a design that minimizes BPT, and a diesel engine manufacturer should strive to develop engine control strategies for raising the exhaust gas temperature to temperatures in excess of BPT whenever the amount of trapped particulates exceeds some threshold that has been predetermined in a suitably appropriate manner, such as by experimentation. Using an engine control to raise exhaust gas temperature in this way is called forced regeneration.
Control of fueling is important in forcing regeneration. A known electronic engine control system comprises a processor-based engine controller that processes data from various sources to develop control data for controlling certain functions of the engine, including the amount and the timing of engine fueling. A typical diesel engine that comprises fuel injectors for injecting fuel into the engine cylinders under control of an engine control system controls both the duration and the timing of each fuel injection to set both the amount and the timing of engine fueling. During an engine cycle, more than one injection of fuel into a cylinder may occur. Pilot injection that precedes a main injection and post-injection that follows a main injection are examples. Proper fueling for initiating CDPF regeneration can be accomplished by controlling injections such as these.